Bill, Wikitree's GEDCOM importer assigns each source in that GEDCOM a number. Actually, it uses the source number that comes along with the GEDCOM. Each GEDCOM assigns a number to each and every source. So, if in your personal family tree program, you have a variety of sources, when you export from the program to a GEDCOM, the export process creates, for examples:
S1234 Vital Records of Massachusetts
S4567 Probate Records of Sussex County, Massachusetts
S8910 1930 US Census... etc. (dpending on how you entered it in your family tree program)
If you look at a wikitree profile created by an uploaded GEDCOM, you'll see reference to these #S1234 source numbers.
Personally, I go through such profiles, and clean them up.
Is there a particular challenge you're facing that you'd like help with?